Software Defined Radio

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Not to be confused with very low power SRD Short Range Devices or SRDs - the European version of Part 15 intentional radiators.

A software defined radio, or SDR, is a radio transmitter or receiver where some functions tradionally performed with RF and IF circuits (e.g. mixers, filters and, especially, modulators/demodulators and detectors) are instead implemented in software running on an associated desktop computer or on an embedded computing device.(1) While the concept of SDR is not new, the rapidly evolving capabilities of digital electronics render practical many processes which used to be only theoretically possible.

SDR technology

A basic SDR receiver consists of a personal computer equipped with a sound card, preceeded by an RF front end which converts radio frequency signals to a frequency range which the sound card can sample and process. Normally two low frequency streams, sampled with local oscillators operating with a 900 phase difeerence, are produced. These are usually called the I and the Q signals. The sound card implements the final stages of signal filtering. Most important, having both I and Q signals fed to the sound card, it is possible to implement an almost endless variety of detectors and demodulators in software.

More advanced SDR receivers, like Perseus SDR from Microtelecom, include a high speed sampler (analog-to-digital converter) and send digitized I and Q signals to the computer using a fast digital link, like USB of firewire.


Software radios are used in military applications, including VHF and UHF tactical radios, in cellular phones, in GPS receivers and are becoming very common in amateur radio and shortwave listening stations. In the long term it is expected that software-defined shall become the dominant technology in radio communications.

You can find an extensive listing of SDRs for numerous applications here


  1. Markus Dillinger, Kambiz Madani, Nancy Alonistioti: "Software Defined Radio: Architectures, Systems and Functions", Wiley & Sons, 2003, ISBN 0-470-85164-3.

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